Field of the Invention
The invention of the present application relates to the general field known as load shifting or peak shaving. More particularly, the invention relates to Thermal Energy Storage (TES) installations employing large tanks wherein a thermal transition zone, often referred to as a thermocline, is developed in a temperature stratifiable liquid in the tank. The thermocline separates a chilled liquid on one side of the thermocline from a relatively warmer liquid on the other side of the thermocline. In the case of water and aqueous solutions, for example, the cooler denser liquid will be situated below the thermocline, while the warmer less dense liquid will be situated above the thermocline. Even more particularly, the invention relates to diffusers disposed in such tanks for the purpose of minimizing internal mixing as liquid is introduced into and/or discharged from the tank, all for the purpose of establishing minimally sized thermoclines and reducing the overall cost of TES tank installations and operations.
Prior Art Background
The principal purpose of a diffuser in a TES tank is to slowly introduce a temperature stratifiable fluid into the tank in such a way that mixing within the tank is negligible. This lack of mixing within the TES tank allows the fluid to stratify and be stored at two different temperatures. The methods used for designing standard prior art octagonal diffusers are fairly straightforward. Such design procedures are detailed in a publication of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) entitled “Design Guide for Cool Thermal Storage” and authored by Charles Dorgan, PE and James Elleson, PE.
Naturally stratified TES tanks take advantage, for example, of the normal differences in the density of water at different temperatures to separate chilled water from warmer water returning from an air handling system, for example. By limiting the inlet and outlet velocities of the water, internal mixing may be minimized whereby buoyancy forces dominate, thus allowing the water to stratify. This allows warm water to be stacked on top of cooler denser water without the need for a physical membrane to separate them. A layer of water, generally referred to as a thermocline, is disposed between and separates warm return water from cold stored water. A major key for optimization of the performance of thermal storage is the design of an internal diffuser adapted to create the thinnest possible thermocline. The operation of a conventional prior art TES installation is illustrated in Prior art FIGS. 15A and 15B. FIG. 15A schematically illustrates the characteristic relative horizontal position of the thermocline 212 during off-peak hours, while FIG. 15B schematically illustrates the characteristic relative horizontal position of the thermocline 212 during peak hours. In each case, the relatively warmer water is shown as small Xs and the relatively colder water is shown as small Os. As is well known to those of ordinary skill in the TES field, depending often upon momentary fluctuations in demand, the thermocline is continuously in motion, up and down in the tank, throughout both the on and off peak hours.
Institutional electrical energy usage, for example, follows a bell curve with peaks during the day and valleys overnight. Local power plants must have the capacity to handle the peak periods. Accordingly, power plants are often oversized so as to meet the high demands during peak periods and are forced to charge demand fees to offset the cost of excess capacity. During periods of low demand (generally in the evenings), plants must be kept running and producing electricity, often with excess capacity. In many cases, energy produced during low demand periods is offered to large commercial users at a reduced rate. TES takes advantage of these low rates by chilling water for the cooling systems at night and storing it in insulated tanks for use during periods of peak demand. This is known as load shifting or peak shaving, depending on the specific geographic location and utility incentives. Often the payback of the initial construction cost of a TES installation can be very short.
As mentioned above, diffusers may be disposed in TES tanks for the purpose of minimizing internal mixing as liquid is introduced into and/or discharged from the tank. The main purpose of the diffuser is to introduce liquid into the tank and remove liquid from the tank at very low velocities, thus eliminating or at least minimizing mixing of the warm liquid and the cold liquid within the tank, whereby both warmer liquid and relatively colder liquid may be stored in the same tank. Proper stratification of the liquid may be achieved only by proper diffuser design so as to achieve the desired low design flow rates.
Many diffuser designs are described in ASHRAE publications, including octagonal and H-shaped diffusers. Currently, many companies involved in designing and constructing TES tanks utilize the octagonal diffuser layout. Such a design is currently a standard in the industry, and is outlined in many ASHRAE publications. One of the shortcomings of this design is that the construction of the octagonal diffuser requires large quantities of piping and pressure fittings. Long lengths of pipe and the corresponding large quantities of fittings may often cause the octagonal diffusers to be overly costly to build and utilize.
As a result of the foregoing, the industry is continually searching for a diffuser design which reduces and minimizes construction and operational costs as well as the thickness of the thermocline. By reducing and/or eliminating mixing of the chilled and warmer liquids, a thinner thermocline with its reduced volume may be developed between the liquids whereby the holding capacity of the tank is improved.
In addition to the foregoing, TES systems may be called upon for use in connection with temperature stratifiable fluids and liquids other than water. For example, see U.S. Pat. No. 5,176,161, col. 1. ll. 24-41, where a number of temperature stratifiable liquids are mentioned. It is to be noted in this regard that in some of the stratifiable liquids, the density of the warmer layer will be greater than that of the colder layer, so in those cases the colder layer will be on top.